1. Technical Field of the Invention
The present invention relates generally to electromagnetic switches. More particularly, the invention relates to an electromagnetic switch for an automotive starter motor, which includes a fixed magnetic core that has a disc portion formed of a stack of base and balance metal sheets.
2. Description of the Related Art
U.S. Pat. No. 6,281,770 B1 discloses an electromagnetic switch which includes a fixed magnetic core that is made in two-part form.
More specifically, referring to FIG. 7, the fixed magnetic core has a base portion 100 and a disc portion 110 that are made separately. The base portion 100 is made to have a boss 130. The disc portion 110 is made by stacking a plurality of thin laminations 120, each of which has a center through-hole. The base and disc portions 100 and 110 are assembled together by press-fitting the boss 130 of the base portion 100 into the center through-holes of the laminations 120 that make up the disc portion 110.
In order to minimize the manufacturing cost, the laminations 120 are generally made up of standard steel laminations which are readily available from the market.
However, in such a case, all the laminations 120 have the same thickness, and thus it is difficult to set the thickness of the disc portion 110, which is the sum of thickness of the laminations 120, to a desired value. Accordingly, in design of the magnetic circuit of the switch, it is difficult to optimize the cross section of the disc portion 110.
Further, in such a case, the press-fit load for each of the laminations 120 is smaller than in the case of a disc portion that is formed of a piece of thick metal sheet. Accordingly, each of the laminations 120 can be detached from the boss 130 of the base portion 100 with a smaller force. Consequently, when the movable magnetic core collides against the fixed magnetic core during operation of the switch, the laminations 120 may be detached from the boss 130 of the based portion 100 due to a mechanical shock caused by the collision.